A compound containing a weakly coordinating anion (i.e., an anion that coordinates only weakly with a cation) is useful in a variety of applications including as an electrolyte and a counter-ion for a catalyst in a variety of organic reactions. Some of the useful catalysts containing a weakly coordinating anion are described by Barbarich, et al. in "LiAl(OC(Ph)(CF.sub.3).sub.2).sub.4 : A Hydrocarbon-Soluble Catalyst For Carbon--Carbon Bond-Forming Reactions", Organometallics, 1996, 15, 3776, which is incorporated herein in its entirety.
Investigations of very reactive metal and nonmetal cations continues to spur the development of new weakly coordinating anions. See, for example, Bochmann, Angew. Chem., Int. Ed. Engl. 1992, 31 1181; Strauss, Chem. Rev. 1993, 93, 927, Strauss, Chemtracts-Inorganic Chem. 1994, 6,1; and Seppelt, Angew. Chem., Int. Ed. Engl. 1993, 32, 1025. One of the most important uses of weakly coordinating anions is to enhance the catalytic activity of metal cations. Two examples that have received considerable attention recently are metallocene-catalyzed olefin polymerization, and lithium-catalyzed Diels-Alder reactions and 1,4-conjugate addition reactions. See Turner, European Patent Appl. No. 277,004, 1988; Pellecchia et al., Makromol. Chem., Rapid Commun. 1992, 13, 265; DuBay et al., J. Org. Chem. 1994, 59, 6898; Saidi et al., Chem. Ber. 1994, 127, 1761; Kobayashi et al., Chem. Lett. 1995, 307; and Arai et al., Angew. Chem., Int. Ed. Engl. 1996, 15, 3776.
Useful anions must not only be weakly coordinating, they must also be stable with respect to oxidation and/or fragmentation in the presence of highly electrophilic cations. In addition, an ideal weakly coordinating anion should have a single negative charge dispersed over a large surface composed of relatively nonpolar bonds to weakly basic atoms such as hydrogen or the halogens. Weakly coordinating anions which conform to many, but not all, of these criteria include B(Ar.sub.f).sub.4.sup.- (A.sub.f =C.sub.6 F.sub.5 or 3,5-C.sub.6 H.sub.3 (CF.sub.3).sub.2), CB.sub.11 H.sub.12-n X.sub.n.sup.- (X=H or I), CB.sub.9 H.sub.10-n X.sub.n.sup.- (X=H, Cl, Br or M(OTeF.sub.5).sub.n.sup.- (n=4, M=B; n=6, M=Nb, Sb)).
All of the anions mentioned above have limitations. Some are too strongly coordinating for specific applications. Some are unstable under the harsh chemical conditions where they would be employed. For example, the fluorinated derivatives of BPh.sub.4.sup.- can react with strongly electrophilic cations, causing (i) cleavage of a C--F bond and formation of a bond between the fluorine atom and the cation or (ii) transfer of a fluoroaryl group to the cation. In either case, the cation is no longer reactive or catalytically active.
Other weakly coordinating anions, such as ClO.sub.4.sup.-, BF.sub.4.sup.-, PF.sub.6.sup.-, SbF.sub.6.sup.-, B(OTeF.sub.5).sub.4.sup.-, and Nb(OTeF.sub.5).sub.6.sup.-, are not thermally and/or hydrolytically stable. In addition, lithium salts of such anions, including LiCF.sub.3 SO.sub.3 and LiPF.sub.6, have low electrical conductivity in organic solvents, especially organic solvents that are stable in the presence of strong reducing agents such as metallic lithium and related lithium-containing battery anode solutions. Moreover, some lithium salts, such as LiPF.sub.6, are known to be unstable and decompose over time.
Still other anions containing boron atoms, and anions containing a carbon atom and a cluster of boron atoms, such as carboranes (e.g., CB.sub.6, CB.sub.9, CB.sub.11) , are not particularly weakly coordinating because the salts formed therefrom contain at most only one fluorine atom which is bonded to a boron atom.
Recently, polyfluorinated carborane anions that are weakly coordinating and are thermally and/or hydrolytically stable have been disclosed in U.S. patent application Ser. No. 09/049,420, filed Mar. 27, 1998.
Despite the recent advances in weakly coordinating anions, there still is a need for a new weakly coordinating anion. There is also a need for a weakly coordinating anion having a high electrical conductivity in an organic solvent.